2 Introductionof the possibilities which have arisen. We describe different strategies for increasing the spectral resolution, for example by measuring nuclear frequencies directly with pulse ENDOR techniques or indirectly by electron spin echo envelope modulation (ESEEM) methods, and explain the concepts needed for the adaptation of existing experiments and the design of new methods for solving application problems.The basic concepts of pulse EPR are closely related to those of pulse nuclear magnetic resonance (NMR) spectroscopy1 l2H'7'. However, the theory developed to describe the dynamics of a spin system in NMR has to be supplemented with EPR-specific modifications. These modifications have their origin in a number of major differences, for example in the relative size of the magnetic interactions and in the relaxation times which are approximately three orders of magnitude shorter in EPR than in NMR. Consequently, the methodology of pulse EPR had to follow its own paths.Pulse EPR spectroscopy is embodied in two textbooks and a number of review articles. The book of Salikhov, Semenov. and Tsvetkov'18', published in Russian in 1976, describes electron spin echo phenomena, whereas the book of Dikanov and Tsvetkov'19', published in 1992. focuscs on ESEEM spectroscopy. A useful early review about electron spin echoes, with special emphasis on the physics of the different relaxation mechanisms, was published in 1972 by Mims'20'. Different fields of pulse EPR spectroscopy are described in edited books'-'M^ and a number of review articles dealing with methodology and applications'26'"'39', and with special topics such as. echo phcnomcna|,t0H42', ESEEM'43'-'5", pulse ENDOR|52H591, relaxation measurements and dynamical processes'601 J6", one-dimensional (1D) and two-dimensional (2D) Fourier transform (FT) EPR162'"'65', high-frequency pulse EPR166'*67', instrumentation168'-'711, and data processing'72' '74', have been published.1.2 A short history of pulse EPRThe historical development of pulse EPR followed quite unusual pathways. Much of the pioneering work in the field was done in the 1960s under the leadership of W. B. Mims at Bell laboratories. In the following decade the field was cultivated only by a small number of research centres, in particular, the Mims group and the group of Yu. D. Tsvetkov in Novosibirsk. This initial shadowy existence of pulse EPR is probably explained by the expensive instrumentation needed and the lack of suitable microwave (m.w.) components and sufficiently fast digital electronics.It was in the 1980s when pulse EPR started to undergo rapid development. In 1982, the first workshop devoted solely to pulse EPR was held in Leiden, The Netherlands. The construction of home-built pulse EPR instruments became less demanding and commercial pulse EPR and ENDOR spectrometers operating at X-band frequencies came on the market'75' (for the different frequency bands sec Table 1.2.1).Interestingly enough, high-field EPR spectroscopy experienced a similar development. For many years the research group under the guidance of Ya. S. Lebedev in Moscow did virtually all the pioneering work. In the 1990s high-field EPR and in particular pulse EPR and pulse ENDOR developed to a new fast advancing field in magnetic resonance spcctro- scopy[66'|67U76,J77'. and a commercial pulse EPR and ENDOR spectrometer operating at W-band has become available'78' '79'.